In measuring the viscosity of a fluid sample, the temperature of the fluid sample is generally controlled by using a thermostat. Water, oil or the like are used as a liquid for heating and cooling the fluid sample, and the temperature of the fluid sample is indirectly controlled through regulation of the temperature of liquid. Therefore, it requires a lot of time to adjust the fluid sample temperature to a desired set point. In addition, when the viscosity of the fluid sample is sequentially measured over a wide temperature range, it may be difficult to estimate the actual temperature of the sample varying with time after changing the set point of the fluid sample temperature.
For example, as described in Kawata, Michio; Viscosity, Revised Edition; Corona Publishing Co., Ltd., Japan; 1983; page 41, in a capillary viscometer, the viscometer is immersed in a liquid bath of a thermostat, and the temperature of the liquid bath is regarded as the temperature of a fluid sample within the viscometer. However, because the temperature of the sample cannot be directly measured in addition to the fact that the heat conductivity of the sample differs with the kind of sample, it takes a long time to correctly determine a relationship between viscosity and temperature of the sample.
Also, as described on pages 109 to 110 of the above technical book, in a rotation viscometer, a sample container is surrounded by a water jacket, and water is forcedly circulated between the jacket and a water bath of a thermostat installed outside the jacket to control the temperature of the sample. In this case, temperature controlling is carried out with respect to the water in the thermostat without feedback of the temperature of the sample to the temperature controlling of the thermostat. It is, therefore, not easy to adjust the sample temperature to a desired set point.